The present invention relates to new alkali metal cyanide granulates based on sodium cyanide or potassium cyanide, whose particles are essentially spherical and which are characterized by extraordinary properties as compared with previously known granulates, including minimal abrasion and a greatly reduced tendency to cake. The present invention also concerns a process for preparing such granulates.
The two alkali metal cyanide salts sodium and potassium cyanide are used to prepare electrolytic baths and hardness salt baths as well as for the synthesis of organic compounds. In addition, sodium cyanide is used in large amounts to recover gold by the cyanide leaching of ores.
Due to their toxicity, the alkali metal cyanides mentioned are preferably handled in the compressed form or as low-dust milling granulates. Sodium and potassium cyanide granulates which are produced commercially on a large scale consist of irregularly shaped particles with a particle size distribution which ranges between, for example, about 0.2 and 4 mm. Compressed products are produced using rotating roller presses or ram presses. The granulates are prepared by crushing the pressed discs using a milling device. The irregular shape of this type of granulate is the reason for the undesired dust formation during handling procedures.
Known NaCN and KCN commercially available compressed products, milling granulates and powders have a strong tendency to agglomerate and cake on the internal surfaces of piping, fittings and equipment made of steel or stainless steel. The tendency to agglomerate and cake increases with decreasing particle size and in particular with increasing humidity and applied load, and makes handling procedures, such as storing in silos, weighing out and transporting, difficult and expensive.
Due to these difficulties, the present inventors looked for a form of supply which did not tend to agglomerate and cake, or at least had a lower tendency to do so. Although NaCN filter cakes which were prepared by known methods could be converted into an approximately spherical granulate and then dried, the resulting NaCN granulate had too low a bulk density, less than 600 g/dm.sup.3, and the resistance to abrasion was unsatisfactory.
A further disadvantage of the previously known sodium and potassium cyanide granulates was the technically complicated process of preparation, which comprised several steps and which started with aqueous NaCN and KCN solutions respectively. In the first step, the alkali metal cyanide is crystallized out of solution, this being performed, for example, in a vacuum crystallizer. The crystallized product is separated from the mother liquor using known techniques. After a single- or multi-stage drying procedure, the alkali metal cyanide powder is compacted to give a compressed product, the latter being crushed to give the granulate mentioned. Thus the process requires not only a large investment in equipment and service personnel, but the evident tendency of NaCN and KCN to cake forces stoppages and reduces the availability of the whole plant.
Glatt Ingenieurtechnik GmbH (Weimar, Germany), in their company document Glatt.RTM. AGT (Feb. 92-3000 D (W)), describe a plant for continuous granulation and drying which is based on the principle of the technique also known as fluidized bed spray granulation (see H. Uhlemann in Chem.-Ing. Technik 62 (1990) no. 10, pp. 822-834) and describe its application to, inter alia, "carbonates and cyanides". However, it is not evident from the company document mentioned whether this refers to alkali metal, alkaline earth metal or heavy metal cyanides or to complex cyanides. Also, neither data on the properties of the cyanide granulates nor the conditions of production of the same can be gathered from the company document.